One use for seismic waves generated by machinery is to image and characterize subterranean structures. Subterranean structures reflect seismic waves, and the reflected seismic waves are detected by machinery for analysis. The inventors have recognized that longitudinal seismic waves generated by machinery travel substantially perpendicular to the earth's surface. The inventors have also recognized that subterranean structures reflect longitudinal seismic waves and in the reflection process convert a substantial portion of the longitudinal seismic waves into transverse seismic waves. The inventors have also recognized that transverse seismic waves travel substantially non-perpendicular to the earth's surface.
The inventors have recognized that conventional seismic wave generators generate transverse and longitudinal seismic waves. Longitudinal seismic waves are frequently referred to as primary waves, or p-waves. Longitudinal waves are primarily compression waves that cause motion of the medium the waves travel through in the direction of wave propagation. Transverse seismic waves are frequently referred to as secondary waves, shear waves, or s-waves. Transverse waves primarily cause motion of the medium the waves travel through in a direction perpendicular to the direction of wave propagation. The inventors also recognized that transverse seismic waves from a seismic wave generator may make it difficult for a seismologist analyzing the reflected seismic returns from the earth, which may include transverse seismic waves, to distinguish transverse waves generated by the seismic source from transverse waves reflected by subterranean structures of potential interest.
Conventional devices for generating seismic waves are generally a detonated explosive charge or gun fired into the bottom of a borehole, a vibroseis vehicle, or a conventional piling driven into the ground. While the conventional devices generally create seismic waves penetrating into the ground, the inventors have recognized numerous drawbacks for imaging subterranean structures using conventional devices. For example, it may be difficult to control the longitudinal and transverse seismic energies created by explosives, guns or vibroseis vehicles. Another example the inventors recognized is that explosives and guns may destroy the mechanical interface between the seismic generator and the earth, thus limiting their use to once or twice at a particular site. Another example is that vibroseis vehicles may not improve the mechanical interface between the transducer plate and the earth, which may make it difficult to propagate energy into the ground.
The inventors have recognized that a conventional piling, such as a pipe or solid rod, may transmit relatively substantial transverse waves when the piling is driven into the ground. The inventors recognized that transverse waves may substantially result from mechanical shear forces created at the interface between the piling sides and the earth as the piling is driven into the ground. Transverse waves may also substantially result from compression waves that are created as a piling is driven into the ground. As noted earlier, transverse waves emanating from a seismic wave source may make subterranean imaging, characterization or analysis difficult and imprecise if a seismologist analyzing the seismic returns from the earth has difficulty distinguishing transverse waves generated by the seismic source from transverse waves reflected by subterranean structures of potential interest.
Thus, the present inventors have recognized the need for a seismic wave generating source that creates seismic waves having a relatively strong longitudinal energy and relatively weak transverse energy.